1. Field of the Disclosure
This disclosure generally relates to explorations for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this disclosure relates to a method and apparatus for processing the image obtained by a resistivity imaging tool in oil-based mud
2. Background of the Art
Resistivity imaging tools have been in widespread use for several years for obtaining resistivity images of walls of boreholes drilled in an earth formation. Oil-based muds must be used when drilling through water soluble formations: an increasing number of present day exploration prospects lay beneath salt layers. Besides reducing the electrical contact between the logging tool and the formation, invasion of porous formations by a resistive, oil-based mud may greatly reduce the effectiveness of typical resistivity imaging devices. This problem is not alleviated by the use of focusing electrodes.
The prior art devices, being contact devices, are sensitive to the effects of borehole rugosity—the currents flowing from the electrodes depend upon good contact between the electrode and the borehole wall. If the borehole wall is irregular, the contact and the current from the electrodes may be irregular, resulting in inaccurate imaging of the borehole. A second drawback is the relatively shallow depth of investigation caused by the use of measure electrodes at the same potential as the pad and the resulting divergence of the measure currents.
Another drawback involving the use of contact devices injecting electrical currents into a wellbore may arise when oil-based muds are used in drilling. Oil-based muds may be used when drilling through water soluble formations as an increasing number of present day exploration prospects lay beneath salt layers. In addition to reducing the electrical contact between the logging tool and the formation, invasion of porous formations by a resistive, oil-based mud may greatly reduce the effectiveness of typical resistivity imaging devices. This problem may not alleviated by the use of focusing electrodes. Standoff changes and leakage currents may degrade the quality of the image produced by a typical resistivity tools. The present disclosure is directed to methods of processing the image to improve the data quality and eliminate systematic and random noise in the image.